Field of Invention
The present invention relates in general to a method for decoding cell ID, and more particularly to a non-coherent method for decoding neighbor cell ID.
Related Art
In conventional mobile cellular system, in order to synchronize the time and frequency between a user equipment (UE) and a base station (eNB), an initial cell search is performed as soon as the UE starts up. In addition, the UE also needs to perform a cell search procedure to obtain a cell ID of the eNB, and then the access between the eNB and the UE can be performed. Taking the Third Generation Partnership Project Long Term Evolution (3GPP LTE) for example, in every cell, two types of synchronization signals will be broadcast for the UE to perform the cell search procedure. One type is the primary synchronization signal (PSS), the other is the secondary synchronization signal (SSS). In the downlink system, the UE performs time and frequency synchronization through PSS and SSS, and uses the two types of synchronization signals to obtain cell identities (Cell IDs).
There are 504 types of cell IDs in the LTE system, including group IDs and sector IDs, used to define cell IDs. The expression for cell ID reads as follows: NIDcell=3NID(1)+NID(2). NID(1) represents a group ID, NID(1)ε{0, 1, . . . , 167}; NID(2) represents a sector ID, NID(2)ε{0, 1, 2}. The above PSS is a Zadoff-Chu sequence, carrying the information of the sector ID NID(2). The above SSS consists of two maximum length sequences (ML-sequences), and the two ML-sequences are produced by two cyclic shifts (m0, m1) carrying the group ID NID(1). The mapping of group ID, ranging from 0 to 167, to a set of cyclic shifts (m0, m1) is one-to-one.
Further, for those mobile UEs to sustain connected to the network, they not only have to search the serving cell in their present location, but periodic neighbor cell search is also required to perform fast and efficient handover for the UEs in motion. However, the synchronization signal from the neighbor cell is generally very weak and seriously interfered by signals from the home cell. As a result, searching for neighbor cell ID becomes very difficult for the UE.
Conventional coherent detection uses the PSS to perform channel estimation and removes the signal component of the serving cell from the received signal, and then, the detection of the group ID and sector ID of the neighbor cell begins. However, the received PSS includes interferences, so the channel estimation is inaccurate and greatly fluctuates, and that causes the reduction of the detection accuracy of the neighbor cell ID. Therefore, before neighbor cell detection, the receiver often uses a channel smoothing filter to further process the detected channel responses, thus increasing the amount of computation in the receiver.